Solid state lighting elements, such as LEDs, are being increasingly employed in lighting devices due to the advantages they offer in terms of energy efficiency and longevity. These advantages have led to a desire to replace traditional bulbs and lamps with lighting devices comprising solid state lighting elements.
However, the colour temperature and ‘sparkle’ qualities of the lighting effect provided by filament bulbs (e.g. halogen bulbs) remain desirable characteristics of such traditional lighting devices. Hence it remains a challenge to design a lighting device using solid state lighting elements which may approximate or mimic the lighting effect provided by traditional lamps/bulbs. Such a lighting device may often employ a plurality of solid state lighting elements because the light output of a single LED may be less than that produced by, for example, traditional compact fluorescent/incandescent lamps. Furthermore, solid state lighting elements generally approximate point sources such that a plurality of solid state lighting elements may be required in order for the lighting device to illuminate in different directions.
A further benefit to use of a plurality of solid state lighting elements is the possibility to produce a lighting effect which may approximate the colour temperature produced by a traditional lamp, such as a halogen bulb. It may be desirable to configure the solid state lighting elements such that the colour temperature decreases with greater degree of dimming in order to more closely approximate the lighting effect of a traditional lamp/bulb. The colour temperature may, for example, be tuned by mixing the light generated by at least two solid state lighting elements configured to emit white light of different spectral composition relative to each other.
Lighting devices employing solid state lighting elements may for example comprise a lens body surrounding an elongate carrier bearing a plurality of solid state lighting elements. The lens body may, in some respects, replicate the shape and appearance of, for example, a traditional halogen bulb; the elongate carrier mimicking the filament of such a bulb.
The lens body may provide a degree of beam shaping of the light produced by the plurality of solid state lighting elements. However, the lens body may result in different beam shaping effects of light emitted from solid state lighting elements depending on where they are mounted on the elongate carrier which may result in poor merging of the light profiles generated by the lens body corresponding to solid state lighting elements mounted on different regions of the elongate carrier. This effect may result in poor mixing of light emitted by the various solid state lighting elements. This can produce image artifacts, e.g. rings, in the overall image produced by the lens body.
This may be particularly noticeable if the solid state lighting elements are configured to emit light (e.g. white light) of different spectral composition relative to each other due to the different colours of the resulting poorly merged light profiles. Colour mixing of light of different spectral compositions (e.g. high and low colour temperature white light) may be used to approximate the ‘warm glow’ lighting effect of dimmed halogen/incandescent light sources. Poor colour mixing resulting from poorly merged light profiles may result in less effective mimicking of this lighting effect by a solid state lighting device.
This beam shaping issue is illustrated by the prior-art lighting device 10 shown in FIG. 1. The lighting device 10 comprises a lens body 11 encapsulating a (gas-filled) cavity 12, with an elongate carrier 15 comprising a plurality of solid state lighting elements 16 extending into the said cavity 12. The solid state lighting elements 16 may be positioned along the length of the elongate carrier 15. The inner surface 13 of the lens body 11 may arch over the elongate carrier 15.
The arrows in FIG. 1 schematically represent the path of (selected) light rays from the solid state lighting elements 16a and 16b mounted at different lengths along the elongate carrier 15. In spite of one of the depicted light rays emitted from solid state lighting element 16a being substantially parallel with respect to the depicted light ray emitted from solid state lighting elements 16b, the respective rays are incident on differently shaped surface portions of the inner surface 13 (differing in terms of how curved they are relative to each other) which may cause these rays to be refracted by the lens body to different extents. Thus FIG. 1 is illustrative of the different beam shaping effect of the lens body 11 on the light emitted by solid state lighting elements 16 depending on their position on the elongate carrier 15. This effect may result in at least partially separated light profiles generated by the lens body corresponding to the respective solid state lighting elements 16a and 16b being visible on a surface (not shown) illuminated by the lighting device 10. These profiles may be perceived as distinct bright areas or rings appearing on the surface corresponding to light emitted from respective solid state lighting elements 16 mounted on different positions of the elongate carrier 15.
This effect may be illustrated with reference to FIG. 10 which shows a simulated lighting effect on a surface illuminated by the prior-art lighting device 10 (not shown in FIG. 10). Pane 50a shows the lighting effect provided by the solid state lighting elements 16b (with solid state lighting elements 16a not emitting). Pane 50b shows the lighting effect provided by the solid state lighting elements 16a (with solid state lighting elements 16b not emitting). It may be seen in pane 50c wherein both 16a and 16b are emitting, that the images corresponding to the respective solid state lighting elements are clearly distinguishable (i.e. are poorly mixed); the illumination of the surface being noticeably dominated by the bright ring corresponding to the light emitted by solid state lighting elements 16a. 
Furthermore, if the solid state lighting elements 16a and 16b are configured to emit light of different spectral composition relative to each other, the colour-mixing of the light emitted by the respective solid state lighting elements may be poor. This effect may result in the appearance of differently coloured bright areas/rings on a surface illuminated by the lighting device 10.